Trim the roast of excess fat (don't worry about the fat marbled in the roast, but anything around the edges or that starts to come off when you tug at it). Slice it into 1" slices. 

Heat 2-3 tablespoons of olive oil in your Instant Pot on the sauté setting. Cut the onion in half and slice. Set aside.

While the oil is heating, sprinkle the roast slices with the steak seasoning and rub it into the meat. When the oil is very hot, arrange as many of the meat slices as possible in a single layer in the bottom of the pan and sear 2-3 minutes. Flip and sear on the other side. If you weren't able to fit all the roast pieces into the pan, remove the seared pieces and sear the remaining roast. Remove from pan. 

Sauté the onion in the pan for 2-3 minutes. Add the smashed or chopped garlic in the last minute of cooking. Add the vinegar (or wine), broth, and seared roast pieces. Sprinkle the onion soup mix and stir to combine everything. Use the manual setting to cook on high for 40 minutes.

When the roast is done cooking, allow the pressure to release naturally (don't turn the valve) for 10 minutes, then use the release valve to release any excess pressure. Shred the roast according to your tastes (some people prefer chunks while others prefer shreds). Transfer to a serving dish and drizzle with the liquid from the Instant Pot and serve.